The present invention relates to a scroll compressor comprising a scroll compression mechanism including a fixed scroll and an orbiting scroll and constructed such that a back pressure chamber filled with a gas refrigerant is provided on a back surface of an end plate at least one of the scrolls and a gas refrigerant pressure in the back pressure chamber presses one of the scrolls against the other of the scrolls, and a refrigerating apparatus.
There is known a scroll compressor comprising a compression mechanism including a fixed scroll, an orbiting scroll, etc. and a drive unit that drives the compression mechanism, wherein the compression mechanism and the drive unit are received in a closed vessel, and such compressor is frequently used in a refrigerating cycle composed of a condenser, an expansion valve, an evaporator, etc. Further, there is known a technology in a refrigerating cycle constructed in such a manner, in which a gas refrigerant downstream of the condenser is injected into the compression chamber to increase a difference in enthalpy across the evaporator to increase a refrigerating capacity, thus improving COP of the refrigerating cycle.
On the other hand, in compressors for refrigeration or cold storage, in which operation at a high pressure ratio is required, or compressors for an air conditioner for cold districts, in which, at the time of heating, operation at a high pressure ratio is required, include one, in which a liquid refrigerant of low temperature on an upstream side of an expansion valve is injected into the compression chamber to decrease discharge gas temperature, thereby suppressing an increase in temperature of a motor winding to enlarge an operating range.
Further, there is known a compressor, in which gas injection and liquid injection are used in the same compressor at need to enable improving COP of a refrigerating cycle and enlarging an operating range.
Scroll compressors constructed such that a back pressure chamber filled with a gas refrigerant is provided on a back surface of a scroll and a gas refrigerant pressure in the back pressure chamber presses one of the scrolls against the other of the scrolls, include one, in which a back pressure chamber is composed of a space filled with suction gas or gas of an intermediate pressure, and a space filled with gas of discharge pressure. In such scroll compressor, the summation of suction gas pressure or intermediate pressure and discharge gas pressure presses one of scrolls against the other of the scrolls, so that the summation of refrigerant gas pressure in the back pressure chamber becomes large under that operating condition of high pressure ratio, in which the discharge gas pressure is high and the suction gas pressure is low.
It is assumed that Ps indicates pressure in the back pressure chamber when the back pressure chamber is put at the suction gas pressure, or Pb indicates pressure in the back pressure chamber when the back pressure chamber is put at the intermediate pressure, and S1 indicates an area of an end plate of a scroll, which bears these pressures. Further, assuming that S2 indicates an area of an end plate of a scroll, which bears a gas pressure of the back pressure chamber filled with discharge gas pressure Pd, a force F1 pressing that scroll, on which refrigerant gas pressure of the back pressure chamber acts, against another scroll is represented by the following formula (1) or (2).F1=Ps·S1+Pd·S2  (1)F2=Pb·S1+Pd·S2  (2)
It is found in the formula (1) that F1 increases and a magnitude thereof is governed by the discharge gas pressure under that operating condition of a high pressure ratio, in which the discharge gas pressure Pd is high and the suction gas pressure Ps is low. Also, since the intermediate pressure Pb becomes also small when the suction gas pressure Ps is small, it is also found in the formula (2) that F1 increases and a magnitude thereof is governed by the discharge gas pressure under that operating condition of a high pressure ratio, in which the discharge gas pressure Pd is high and the suction gas pressure Ps is low. In particular, since a pressure bearing area S2, on which the discharge gas pressure Pd acts, tends to increase in the back pressure chamber, in which a sealing material seals a space filled with suction gas or gas of intermediate pressure, and a space filled with gas of discharge pressure, a pressing force F1 is governed by the discharge gas pressure Pd and becomes hard to be influenced by the suction gas pressure Ps and the intermediate pressure Pb.
On the other hand, a force F2 generated by an internal pressure in a compression chamber, which is defined by a fixed scroll and an orbiting scroll, acts in a reverse direction to the pressing force F1. Assuming that compression process comprises an adiabatic change with a polytropic exponent k being constant, internal pressure P in the compression chamber is represented from the relationship pVK=constant by the following formula (3)P=(Vmax/V)k·Ps  (3)where V indicates a volume of the compression chamber and Vmax indicates a maximum confined volume just after confinement is started.
Further, assuming that Smin indicates a pressure bearing area of the compression chamber, on which the discharge pressure just after termination of compression acts, a force F2 generated by the internal pressure is represented by the formula (4)F2=∫Pds+Pd·Smin =Ps·Vmaxk∫(1/Vk)ds+Pd·Smin   (4)
It is found from the formula (4) that the force (separating force) F2 generated by the internal pressure becomes small since a value of a first term in the formula (4) becomes small when the suction gas pressure Ps becomes small.
A net force F3 pressing one of scrolls against the other of the scrolls becomes a difference (F3=F1−F2) between the pressing force F1 by pressure in the back pressure chamber and the separating force F2 generated by the internal pressure, and this relationship is shown in FIG. 2. It is seen from FIG. 2 that a net force F3 pressing that scroll, on which the refrigerant gas pressure of the back pressure chamber acts, against the other of the scrolls becomes excessively large under that operating condition of a high pressure ratio, in which the discharge gas pressure Pd is high and the suction gas pressure Ps is low. Therefore, there is caused a problem that, under the operating condition of a high pressure ratio, a contact surface pressure at tip ends of the scrolls becomes excessively large, and wear and galling are generated on the tip ends of the scrolls.